diff --git a/README.md b/README.md
index df026f7..c34ae3b 100644
--- a/README.md
+++ b/README.md
@@ -166,6 +166,7 @@ I'm currently working on [Analytics-Zoo](https://github.com/intel-analytics/anal
 | 482 | [License Key Formatting](https://leetcode.com/problems/license-key-formatting/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/482_License_Key_Formatting.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/482_License_Key_Formatting.java) | String processing, lower and len % K, O(n) and O(n) |
 | 485 | [Max Consecutive Ones](https://leetcode.com/problems/max-consecutive-ones/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/485_Max_Consecutive_Ones.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/485_Max_Consecutive_Ones.java) | Add one when encounter 1, set to 0 when encounter 0, O(n) and O(1) |
 | 509 | [Fibonacci Number](https://leetcode.com/problems/fibonacci-number/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/509_Fibonacci_Number.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/509_Fibonacci_Number.java) | 1. Recursive, O(n) <br>2. DP with memo, O(n). Note that N<=30, which means that we can keep a memo from 0 to 30. |
+| 523 | [Continuous Subarray Sum](https://leetcode.com/problems/continuous-subarray-sum/description/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/523_Continuous_Subarray_Sum.py) | O(n) solution using dict() |
 | 538 | [Convert BST to Greater Tree](https://leetcode.com/problems/convert-bst-to-greater-tree/description/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/538_Convert_BST_to_Greater_Tree.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/538_Convert_BST_to_Greater_Tree.java) | Right first DFS with a variable recording sum of node.val and right.val. 1. Recursive.<br>2. Stack 3. Reverse Morris In-order Traversal |
 | 541 | [Reverse String II](https://leetcode.com/problems/reverse-string-ii/solution/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/541_Reverse_String_II.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/541_Reverse_String_II.java) | Handle each 2k until reaching end, On(n) and O(n) |
 | 543 | [Diameter of Binary Tree](https://leetcode.com/problems/diameter-of-binary-tree/description/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/543_Diameter_of_Binary_Tree.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/543_Diameter_of_Binary_Tree.java) | DFS with O(1) for max answer |
diff --git a/cpp/2553_Separate_the_Digits_in_an_Array.cpp b/cpp/2553_Separate_the_Digits_in_an_Array.cpp
new file mode 100644
index 0000000..e88e8c6
--- /dev/null
+++ b/cpp/2553_Separate_the_Digits_in_an_Array.cpp
@@ -0,0 +1,19 @@
+class Solution {
+public:
+    vector<int> separateDigits(vector<int>& nums) {
+        vector<int> answer;
+        int n;
+        for( int i=0; i < nums.size(); i++){
+            n=nums[i];
+            vector<int> temp;
+            while( n>0 ){
+                temp.push_back(n%10);
+                n = n / 10;
+            }
+            for(int j= temp.size()-1; j>=0; j--){
+                answer.push_back(temp[j]);
+            }
+        }
+        return answer;
+    }
+};
diff --git a/java/006_ZigZag_Conversion.java b/java/006_ZigZag_Conversion.java
index ea64188..97e4b1a 100644
--- a/java/006_ZigZag_Conversion.java
+++ b/java/006_ZigZag_Conversion.java
@@ -1 +1,33 @@
-//TODO
\ No newline at end of file
+//006_ZigZag_Conversion.java
+class Solution {
+    public String convert(String s, int numRows) {
+        if(numRows==1) {
+            return s;
+        }
+
+        String answer = "";
+        String[] str_array = new String[numRows];
+
+        for(int i=0;i<numRows;i++){
+            str_array[i]="";
+        }
+
+        int mod = 2*numRows-2;
+
+        for(int i=0;i<s.length();i++){
+            char c = s.charAt(i);
+            int index = i%mod;
+            if(index >= numRows) {
+                index = 2*(numRows-1) - index;
+            }
+            str_array[index]+=c;
+        }
+
+        for(int i=0;i<numRows;i++){
+            answer += str_array[i];
+        }
+
+        return answer;
+    }
+}
+
diff --git a/java/015_3Sum.java b/java/015_3Sum.java
new file mode 100644
index 0000000..cc0a069
--- /dev/null
+++ b/java/015_3Sum.java
@@ -0,0 +1,47 @@
+//015. 3Sum
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        //create result list to store i,j,k
+        List<List<Integer>> result = new LinkedList<List<Integer>>();
+        
+        //sorting nums
+        Arrays.sort(nums);
+        
+        for (int i = 0; i < nums.length - 2; i++) {
+
+            int left = i + 1;
+            int right = nums.length - 1;
+
+            if (i > 0 && nums[i] == nums[i-1]) {
+                continue; //if nums have same numbers, just check one time.
+            } 
+            
+            while (left < right) {
+                int sum = nums[left] + nums[right] + nums[i];
+                
+                if (sum == 0) {
+                    //if sum == 0, store i,j,k
+                    result.add(Arrays.asList(nums[i], nums[left], nums[right]));
+                    left++; //check anoter case
+                    right--;
+                    //if next number == now number
+                    while (nums[left] == nums[left - 1] && left < right) {
+                        left++;
+                    }
+                    while (nums[right] == nums[right + 1] && left < right) {
+                        right--;
+                    } 
+                } else if (sum > 0) {
+                    //if sum > 0, right--;
+                    right--;
+                } else {
+                    //if sum < 0, left++;
+                    left++;
+                }
+            }
+        }
+        
+        return result; //return result list
+    }
+}
+ 
diff --git a/java/026_Remove_Duplicates_from_Sorted_Array.java b/java/026_Remove_Duplicates_from_Sorted_Array.java
new file mode 100644
index 0000000..13f03b1
--- /dev/null
+++ b/java/026_Remove_Duplicates_from_Sorted_Array.java
@@ -0,0 +1,16 @@
+//026. Remove Duplicates from Sorted Array
+class Solution {
+    public int removeDuplicates(int[] nums) {
+        int index = 1;
+
+        for (int i = 0; i < nums.length - 1; i++) {
+            if (nums[i] != nums[i + 1]) {
+                nums[index] = nums[i + 1];
+                index++;
+            }
+        }
+
+        return index;
+    }
+}
+
diff --git a/java/039_Combination_Sum.java b/java/039_Combination_Sum.java
new file mode 100644
index 0000000..7a54dd3
--- /dev/null
+++ b/java/039_Combination_Sum.java
@@ -0,0 +1,29 @@
+//039_Combination_Sum
+class Solution {
+    List<List<Integer>> answer = new ArrayList<List<Integer>>();
+
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        int clen =candidates.length;
+        for (int i = 0; i < clen; i++) {
+            List<Integer> tlist = new ArrayList<Integer>();
+            tlist.add(candidates[i]);
+            backtracking(candidates, i, 1, (target - candidates[i]), tlist);
+        }
+        return answer;
+    }
+    private void backtracking(int[] candidates, int index, int tsize, int target, List<Integer> temp) {
+        if (target == 0) {
+            answer.add(new ArrayList(temp));
+            return;
+        }
+        
+        for (int i = index, len = candidates.length; i < len; i++) {
+            if (candidates[i] <= target) {
+                temp.add(candidates[i]);
+                backtracking(candidates, i, (tsize + 1), (target - candidates[i]), temp);
+                temp.remove(tsize);
+            }
+        }
+    }
+}
+
diff --git a/java/049_Group_Anagrams.java b/java/049_Group_Anagrams.java
new file mode 100644
index 0000000..a787625
--- /dev/null
+++ b/java/049_Group_Anagrams.java
@@ -0,0 +1,42 @@
+class Solution {
+    public List<List<String>> groupAnagrams(String[] strs) {
+        int[][] alphabets = new int[strs.length]['z' - 'a' + 1];
+        
+        for(int i = 0; i < strs.length; i++) {
+            String str = strs[i];
+            
+            for(int j = 0; j < str.length(); j++) 
+                alphabets[i][str.charAt(j) - 'a']++;
+        }
+        
+        boolean[] visited = new boolean[strs.length];
+        
+        List<List<String>> answer = new ArrayList<>();
+        
+        for(int i = 0; i < strs.length; i++) {
+            if(visited[i]) continue;
+            
+            List<String> list = new ArrayList<>();
+            
+            for(int j = i; j < strs.length; j++) {
+                if(visited[j]) continue;
+                if(isAnagram(alphabets[i], alphabets[j])) {
+                    list.add(strs[j]);
+                    visited[j] = true;
+                }
+            }
+            
+            answer.add(list);
+        }
+        
+        return answer;
+    }
+    
+    public boolean isAnagram(int[] arr1, int[] arr2) {
+        for(int i = 0; i < arr1.length; i++) {
+            if(arr1[i] != arr2[i])
+                return false;
+        }
+        return true;
+    }
+}
diff --git a/java/088_Merge_Sorted_Array.java b/java/088_Merge_Sorted_Array.java
new file mode 100644
index 0000000..95c7c8e
--- /dev/null
+++ b/java/088_Merge_Sorted_Array.java
@@ -0,0 +1,29 @@
+class Solution {
+    //we are being given two int arrays and two int variables that state the number of elements in each array, respectively
+    public void merge(int[] nums1, int m, int[] nums2, int n) {
+        //I am using a counter so that I can iterate through the second array inside the for loop
+        int counter = 0;
+        //We know that nums1 is of the size n + m, so we can add all the elements to the array and sort them later
+        //For loop adds all values of nums2 to the end of nums1
+        for(int i=m; i<nums1.length; i++){
+            nums1[i] = nums2[counter++];
+        }
+        //Now that all of the elements are in the array, we can begin the sort process
+        //Though we could use the Arrays.sort function to quickly sort the array, I wanted to implement a sorting algorithm myself
+        //I am implementing what's called a "Bubble Sorting Algorithm" to sort this array
+        //This should work best in our scenario as we don't have to work with a large list of numbers
+        for(int i=0; i<nums1.length; i++){
+            for(int j=0; j<nums1.length-i-1; j++){
+                if(nums1[j] > nums1[j+1]){
+                    int temp = nums1[j+1];
+                    nums1[j+1] = nums1[j];
+                    nums1[j] = temp;
+                }
+            }
+        }
+        //The following function simply prints out everything that is contained within our num1 array
+        for(int i=0; i<nums1.length; i++){
+            System.out.println(nums1[i]);
+        }
+    }
+}
\ No newline at end of file
diff --git a/java/219_Contains_Duplicate_II.java b/java/219_Contains_Duplicate_II.java
new file mode 100644
index 0000000..f88a1a9
--- /dev/null
+++ b/java/219_Contains_Duplicate_II.java
@@ -0,0 +1,44 @@
+import java.util.*;
+
+class Solution {
+  /*
+   * I have to save indice for each index.
+   * So I use the HashMap<Integer, List<Integer>>
+   */
+
+    public boolean containsNearbyDuplicate(int[] nums, int k) {
+        HashMap<Integer, List<Integer>> map = new HashMap<>();
+        
+        for(int i = 0; i < nums.length; i++) {
+            if(!map.containsKey(nums[i])) {
+                map.put(nums[i], new ArrayList<>());
+            }
+            map.get(nums[i]).add(i);
+        }
+        
+		// use Iterator to find appropriate two indice.
+		// Each list guarantee ascending.
+		// So list.get(i) and list.get(i + 1) is minimum.
+        Iterator<Integer> keys = map.keySet().iterator();
+        boolean answer = false;
+        
+        while(keys.hasNext()) {
+            int key = keys.next();
+            List<Integer> list = map.get(key);
+            
+            if(list.size() < 2) continue;
+            
+            for(int i = 1; i < list.size(); i++) {
+                int a = list.get(i - 1);
+                int b = list.get(i);
+                
+                if(b - a <= k) {
+                    answer = true;
+                    break;
+                }
+            }
+            if(answer) break;
+        }
+        return answer;
+    }
+}
diff --git a/java/442_Find_All_Duplicates_in_an_Array.java b/java/442_Find_All_Duplicates_in_an_Array.java
new file mode 100644
index 0000000..b12d6f5
--- /dev/null
+++ b/java/442_Find_All_Duplicates_in_an_Array.java
@@ -0,0 +1,17 @@
+class Solution {
+    public List<Integer> findDuplicates(int[] nums) {
+        int n = nums.length;
+
+        List<Integer> ans = new ArrayList<>();
+
+        for(int i=0;i<n;i++) {
+            int possibleVal = Math.abs(nums[i]);
+
+            if(nums[possibleVal - 1] < 0) {
+                ans.add(possibleVal);
+            }
+            nums[possibleVal - 1] = -1 * nums[possibleVal - 1];
+        }
+        return ans;
+    }
+}
diff --git a/java/787_Cheapest_Flight_Within_K_Stops.java b/java/787_Cheapest_Flight_Within_K_Stops.java
new file mode 100644
index 0000000..9355cf9
--- /dev/null
+++ b/java/787_Cheapest_Flight_Within_K_Stops.java
@@ -0,0 +1,41 @@
+class Solution {
+    //using bellman ford
+    
+    public void computePrice(int[][]flights, int[] prices, int [] temp){
+        for(int[] flight: flights){
+            int u = flight[0];
+            int v = flight[1];
+            int price = flight[2];
+            
+            if(prices[u] != Integer.MAX_VALUE){
+                if(prices[u] + price < temp[v]){
+                    temp[v] = prices[u] + price;
+                }
+            }
+        }
+    }
+    
+    public void copyTempToPrice(int[] prices, int[] temp){
+        for(int i=0; i<prices.length; i++){
+            prices[i] = temp[i];
+        }
+    }
+    
+    public int findCheapestPrice(int n, int[][] flights, int src, int dst, int k) {
+        int[] prices = new int[n];
+        int[] temp = new int[n];
+        
+        Arrays.fill(prices, Integer.MAX_VALUE);
+        Arrays.fill(temp, Integer.MAX_VALUE);
+        
+        prices[src] = 0;
+        temp[src] = 0;
+        
+        for(int i=0; i<=k; i++){
+            computePrice(flights, prices, temp);
+            copyTempToPrice(prices, temp);
+        }
+        
+        return prices[dst] == Integer.MAX_VALUE ? -1 : prices[dst];
+    }
+}
\ No newline at end of file
diff --git a/python/001_Two_Sum.py b/python/001_Two_Sum.py
index e4bd58a..52399a9 100644
--- a/python/001_Two_Sum.py
+++ b/python/001_Two_Sum.py
@@ -58,9 +58,4 @@ def twoSum(self, nums, target):
                 begin += 1
             else:
                 end -= 1
-
-
-if __name__ == '__main__':
-    # begin
-    s = Solution()
-    print s.twoSum([3, 2, 4], 6)
+                
diff --git a/python/007_Reverse_Integer.py b/python/007_Reverse_Integer.py
index a125d47..19aeba8 100644
--- a/python/007_Reverse_Integer.py
+++ b/python/007_Reverse_Integer.py
@@ -1,39 +1,34 @@
 class Solution:
-    # @return an integer
+    def reverse(self, x):
+        # https://leetcode.com/problems/reverse-integer/
+#        flag = True if x < 0 else False
+#       if flag:
+#           x = -x
+#       x = str(x)[::-1]
 
-    # def reverse(self, x):
-    #     max_int = 2147483647
-    #     if x == 0:
-    #         return 0
-    #     isPos = True
-    #     if x < 0:
-    #         x *= (-1)
-    #         isPos = False
-    #     ltemp = []
-    #     while x != 0:
-    #         temp = x % 10
-    #         ltemp.append(temp)
-    #         x /= 10
-    #     result = 0
-    #     # the main solution
-    #     for t in ltemp:
-    #         result = result * 10 + t
-    #     if result > max_int:
-    #         result = 0
-    #     if isPos:
-    #         return result
-    #     else:
-    #         return -1 * result
+#       if flag:
+#           x = "-" + x
 
-    def reverse(self, x):
-        # Note that in Python -1 / 10 = -1
-        res, isPos = 0, 1
+#       value = 2 ** 31
+#       x = int(x)
+#       if -value <= x < value:
+#           return x
+#       return 0
+        
+        is_neg = False
         if x < 0:
-            isPos = -1
-            x = -1 * x
-        while x != 0:
-            res = res * 10 + x % 10
-            if res > 2147483647:
-                return 0
-            x /= 10
-        return res * isPos
\ No newline at end of file
+            x = -x
+            is_neg = True
+
+        res = 0
+        while x > 0:
+            res *= 10
+            res += x % 10
+            x //= 10
+        if is_neg:
+            res = -res
+
+        if res < -2**31 or res > 2**31-1:
+            return 0
+        return res
+    
\ No newline at end of file
diff --git a/python/523_Continuous_Subarray_Sum.py b/python/523_Continuous_Subarray_Sum.py
new file mode 100644
index 0000000..d37ceeb
--- /dev/null
+++ b/python/523_Continuous_Subarray_Sum.py
@@ -0,0 +1,20 @@
+class Solution:
+    def checkSubarraySum(self, nums: List[int], k: int) -> bool:
+        # remeainders[0] = 0 is for when x == 0
+        remainders = dict()
+        remainders[0] = 0
+        pre_sum = 0
+
+        for idx, item in enumerate(nums):
+            pre_sum += item
+            remaind = pre_sum % k
+
+            # remainder doesnt exist then it has to be init
+            # if it exists, then check the prev one has the same remainder
+            if remaind not in remainders:
+                remainders[remaind] = idx + 1
+            elif remainders[remaind] < idx:
+                return True
+
+        return False
+    

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